Superconductivity is the property whereby certain materials have the ability to lose both electrical resistance and magnetic permeability at extremely low temperatures. The commercial feasibility of a given superconducting material is to a large extent dependent on its critical temperature (T.sub.c), i.e., the temperature below which the material performs as a superconductor. Superconducting materials are presently known having a T.sub.c as high as 125K. Among known superconducting materials is the family of ceramic oxides having a perovskite crystal structure and the general formula R.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-x, where R is a rare earth metal and x is between about 1 and 0. The rare earth metals are defined as the lanthanide elements 57 to 71, scandium, and yttrium. Compounds having the above-recited formula are colloquially known as "1-2-3" compounds because of the approximate ratio of R to barium to copper. Among other known ceramic oxide families which have superconducting properties are those based on compounds of Bi-Sr-Ca-Cu-O and Tl-Sr-Ca-Cu-O, each of which may have several phases.
Superconductors are commonly utilized as coatings on metallic substrates which act as a support for the superconductor. There are many different methods for depositing the superconductor coating on the substrate including powder deposition, high and low pressure vapor deposition, and liquid deposition methods. Exemplary vapor deposition methods are sputtering, thermal evaporation, laser ablation and chemical vapor deposition. Exemplary liquid deposition methods are sol-gel techniques using metal organic compounds.
All of the above-recited deposition methods commonly follow the same basic procedure of distributing the superconducting material over the substrate and heating the material to a temperature which effects a high level of atomic mobility in the material. This procedure may be performed in separate sequential steps or it may be performed simultaneously in a single step. In any case, when a high level of atomic mobility is achieved in the superconducting material, proper grain growth is stimulated within the material which imparts the desired superconducting properties thereto.
By way of illustration, a powder deposition method is performed by first distributing the powder onto the substrate in a porous undensified state. An elevated level of atomic mobility is then achieved in the powder coating by rapidly sintering the coating at as high a temperature as possible which does not substantially melt the powder, thereby avoiding formation of phases which would interfere with the superconductivity properties of the coating. Sintering of the powder stimulates grain growth and produces a densified coating which adheres to the substrate and desirably possesses superconducting properties.
Sintering, however, can also detrimentally result in contamination of the coating by the substrate which ultimately diminishes the superconducting properties of the coating. The high atomic mobility of the powder exhibited during sintering enables diffusion of contaminants into the superconductor from the substrate, particularly if any liquid phase is present. Even trace levels of such contaminants in the superconductor can decrease the critical temperature of the superconductor, thereby substantially diminishing the effectiveness thereof.
Substrate contamination of the superconductor coating is a problem with virtually all known superconductor deposition methods, wherein high atomic mobility is required to achieve proper grain growth. As described above, such methods include powder, vapor, and liquid deposition methods. Accordingly, the objects of the present invention set forth below, and the description provided thereafter, apply to coated superconductor structures produced by these known deposition methods generally.
It is therefore an object of the present invention to inhibit the contamination of a superconductor coating by a substrate supporting the same. More specifically, it is an object of the present invention to provide a diffusion barrier between a superconductor coating and a substrate supporting the same, thereby inhibiting diffusion of contaminants into the superconductor from the substrate, particularly during deposition of the superconductor coating onto the substrate when the coating exhibits a high level of atomic mobility. It is another object of the present invention to provide an effective diffusion barrier which does not react with the superconductor to form non-superconducting phases. Finally, it is an object of the present invention to provide a diffusion barrier which further serves as a parting layer, thereby enabling support of a superconductor coating by a substrate having a different coefficient of thermal expansion than the superconductor coating.